mmc: sdhci-of-esdhc: add peripheral clock support

eSDHC could select peripheral clock or platform clock as clock source by
the PCS bit of eSDHC Control Register, and this bit couldn't be reset by
software reset for all. In default, the platform clock is used. But we have
to use peripheral clock since it has a higher frequency to support eMMC
HS200 mode and SD UHS-I mode. This patch is to add peripheral clock support
and use it instead of platform clock if it's declared in eSDHC dts node.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>